For example, in a tap water treatment for obtaining drinking water or industrial water from natural water sources such as seawater, river water, lake and marsh water and underground water, which are suspended water, a sewage treatment for treating domestic drainage such as sewage water to produce recycled water and obtain dischargeable clarified water, or a step of removing insoluble components from a saccharified solution in the production of a saccharified solution, a solid-liquid separation operation (clarification operation) is required so as to separate and remove suspended matter. By such a clarification operation, with respect to the tap water treatment, suspended matter (e.g., clay, colloid, bacteria) derived from natural water sources as suspended water are removed; with respect to the sewage treatment, suspended matter (e.g., sludge) in treated water after biological treatment (secondary treatment) with suspended matter, activated sludge, etc. in sewage water are removed; and with respect to the production of a saccharide solution from liquid starch, insoluble components having not been degraded by an enzyme are removed. For the clarification operation of a saccharified solution, a diatomaceous earth filtration method has been heretofore performed, but in recent years, a membrane filtration method is spreading in place of such a method.
Conventionally, these clarification operations have been performed mainly by a pressure flotation method, a precipitation method, a sand filtration method, a coagulation, sedimentation and sand filtration method, a diatomaceous earth filtration method, etc., but recently, a membrane filtration method is spreading instead of these methods. The advantage of the membrane filtration method includes, for example, (1) the clarification level of the obtained water quality is high and stable (safety of the obtained water is high); (2) the installation space of a filtration apparatus can be small; and (3) an automatic operation is easy. For example, in a pretreatment of seawater desalination reverse osmosis filtration, a membrane filtration method is used as a substitute for or a latter step of the pressure flotation method so as to further improve the water quality of treated water subjected to pressure flotation treatment. In the clarification operation by such membrane filtration, a flat membrane or a hollow-fiber porous ultrafiltration or microfiltration membrane, having an average pore size of several nm to several hundred nm, is used.
In this way, the clarification operation by the membrane filtration method has many advantages that the above-described conventional pressure flotation method, sand filtration method, etc. do not have, and its spread to seawater desalination pretreatment, etc. is being encouraged as a substitute for or a complementary means to the conventional methods. In addition, an organic membrane composed of a resin described in Patent Literature 1 below is often used as the porous membrane.